In a gas turbine power generating plant, compressed air compressed by driving of a compressor provided coaxially with a gas turbine and fuel are introduced into a combustor to be burnt in a combustion chamber in a combustor liner. A high-temperature combustion gas generated by the combustion is introduced into a turbine part composed of stator blades and rotor blades through a transition piece to be expanded to thereby rotationally drive the rotor blades. In the gas turbine power generating plant, by using kinetic energy created by the rotational driving, a power generator, and so on are rotationally driven to perform power generation.
A conventional transition piece has a double-shell structure composed of an inner duct and an outer duct provided around an outer periphery of the inner duct. One end of the inner duct is coupled to a combustor liner in a cylindrical shape, and the other end of the inner duct is coupled to a stator blade at a first stage of a turbine. Thus, the shape of a cross section, of a combustion gas channel in the inner duct, perpendicular to a flowing direction of a combustion gas changes from a circular shape to a sector of annular shape. The outer duct is also formed into a shape corresponding to the shape of the inner duct.
The inner duct has the high-temperature combustion gas flow through the inside thereof, and thus is formed of a Ni-base superalloy, and further has a cooling structure. In the outer duct in the transition piece of a typical gas turbine on order of 1300° C., a plurality of impingement cooling holes through which part of air discharged from the compressor is ejected and made to impinge onto/on an outer surface of the inner duct as cooling air are formed over the entire surface.
As above, the inner duct of the conventional transition piece is formed of a Ni-base superalloy, and is cooled by the cooling air. However, when a base material increases in temperature locally while the gas turbine is in operation, damage such as reduction in thickness due to oxidation or the like, cracks due to thermal fatigue, and creep voids and cracks due to creep damage is thereby caused in the inner duct.
The above damage is repaired by welding or the like at the time of periodical inspection, and the repaired transition piece is used continuously. However, as its continuous employment time is prolonged, a range of the damage tends to spread. The creep voids due to material deterioration are formed over a large area, and are found even in the inside of the base material, for example.
In the case when repairing is performed over a large area, a local heat input amount increases in the weld repairing. Thus, deformation of the inner duct of the transition piece having a thin thickness structure is caused to make the repairing impossible to be performed, and thus the transition piece is sometimes disposed of. Further, with respect to the creep voids in the base material, it is not possible to pinpoint a range where the creep voids are generated, and the transition piece is employed in a state where the creep voids remain in the base material, thus being at high risk of being led to destruction.
The reduction in thickness due to high-temperature oxidation progresses in proportion to an employment period of the transition piece. Then, when the thickness of the inner duct of the transition piece falls below an allowable thickness, the transition piece has a possibility to be led to destruction. With respect to the reduction in thickness, a thickness-reduced portion can be built-up by welding, but an area to be welded is increased, and thus deformation is caused.
In order to avoid problems such as the deformation in the above-described weld repairing, repairing by diffusion brazing is also considered.
The above-described conventional repairing by diffusion brazing, as compared to the weld repairing, can avoid the problems such as the deformation of the base material, but has a difficulty in being applied to large area repairing for, for example, the reduction in thickness, creep void, and so on.